1. Field of the Invention
The present invention relates to electron field emission devices (cold cathodes), vacuum microelectronic devices and field emission displays which incorporate cold cathodes and methods of making and using same. More specifically, the invention relates to cold cathode devices comprising electron emitting structures grown directly onto a growth surface on a substrate material. The invention also relates to patterned precursor substrates for use in fabricating field emission devices and methods of making same and also to catalytically growing other electronic structures, such as films, cones, cylinders, pyramids or the like, directly onto substrates.
2. Description of Related Art
Several publications and patents are referenced in this application within parentheses. These references describe the state of the art to which this invention pertains, and are incorporated herein by references.
Various types of field emitting devices (cold cathodes) have been proposed. Unlike thermal emission devices, which rely on high temperatures to enable a fraction of the free electrons in the emitting material to overcome the barrier of the work function and be emitted, field emission devices rely on a physical phenomenon which has been explained as electrons tunneling from a surface state to a vacuum state when a sufficient electric field is applied to the emitting surface. A typical micro fabricated field emission cold cathode consists of an emitting structure such as a cone with a sharp tip as the electron emitter and an extraction electrode which creates the field that pulls the electrons from the emitting structure. The base of the cone is typically on a conductive surface of the substrate. Usually, the emitting structure is inside a cavity or opening in a dielectric film on the substrate and the extraction electrode is located on top of the dielectric film and proximate to the cavity to produce the field at the emitting surface. The separation between the tip and extraction electrode is on the order of micrometers or less. When the voltage of the extraction electrode is biased sufficiently positively with respect to that of the emitting structure, field emission occurs at the tip without any additional thermal energy.
Some field emission devices are fabricated using technologies developed for microelectronics. The emitter tips are typically fabricated onto a substrate by evaporation, etching or oxidation. Most field emission devices use silicon or molybdenum cones as the electron emitter structures. Field emission devices comprising these cones and also utilizing a gate structure to supply the field potential are usually limited to one emitting tip per gate opening because of the cone structure and methods used for forming these devices.
Manufacture of the cone emitting structures requires sophisticated lithographic and fabrication equipment to form high yield, high-density complex cold cathode structures at low cost. The metal cone emitters are subject to contamination since the surfaces of these cones are reactive. Accordingly, the emitting structures become contaminated in poor vacuums and require cleaning. For example, the metal cone tips become oxidized, and are cleaned by exposing the tips to hydrogen plasma. The metal tip cones also have poor stability of electron emitting yield upon turn-on and require an appreciable "break in" time. Additionally, these materials have high work functions requiring high potentials to attain any given electron emission yield.
An alternative type of cold cathode consists of an electron emitting whisker or fiber, instead of a cone, and an extraction electrode to apply a sufficient electric field to the tip. The tips are made from thin wires (whiskers) of high-melting-point metals, such as tungsten and molybdenum, metal carbides, silicon carbide, or carbon fibers.
Because of the large fiber size and the manufacturing method, these fiber devices typically have large physical dimensions. The electron emitters are first prepared by forming the emitting material into the desired physical geometry, and then mechanically attaching the structure to the substrate. The. separation between the mounted emitter tip and the extraction electrode is on the order of millimeters or larger.
Carbon fibers have been used as field emitting structures. The carbon fibers appear to be more stable than the metal cone structures and do not contaminate easily under normal working conditions. Additionally, certain carbon structures have a low work, function allowing electron emission in low electric fields. Furthermore, carbon fibers also appear to be more robust, i.e. the electron yields over time have a higher stability.
Ex-polymer carbon fibers have been proposed as emitter structures. The ex-polymer carbon fibers are formed from organic precursors. The precursors are extruded into polymeric fibers and the fibers are stabilized by heating in air (200-350.degree. C.), carbonized by heating to about 1000.degree. C., and graphitized by heating to 3000.degree. C. in an inert atmosphere. Among the ex-polymer carbon fibers are PAN fibers which are formed from polyacrylonitrile, a preferred polymeric precursor.
In the above mentioned prior art, the carbon fibers would be formed separately and subsequently attached to the field emission substrate. The prior art carbon fiber emitters typically have diameters of about 7 .mu.m and are usually first sharpened to decrease the radius curvature of the tips and enhance the electric field. The fibers are then mechanically attached to the field emission device. The distance between the carbon fiber emitter tip and the extraction electrode is usually on the order of at least 1 mm (see, for example, J. J. Lambe, U.S. Pat. No. 4,728,851).
Methods proposed for attaching the carbon fibers include mechanically attaching the fiber by partially melting the surface of the field emitting substrate, inserting the fiber and subsequently cooling the structure to secure the fiber. Another method would utilize an adhesive to glue the fibers onto the surface of the field emitting substrate. The adhesive typically comprises epoxy and/or metallic compositions.
The proposed methods of attaching the fibers are highly disadvantageous since each requires the separate steps of forming the carbon fibers and subsequently attaching the fibers to the field emission substrates. For practical reasons, these methods require the use of large carbon fibers since the attachment requires mechanically handling and manipulating the fibers. Since the size of the fibers is large, these field emitting structures often require "sharpening" of the emitting tips to create a field-enhancing morphology. Not only do the steps of mechanically attaching and sharpening the emitter add cost to the production of the cold cathode, they also limit the ability to fabricate dense emitter patterns. Fibers that are too small tend to break during handling and are difficult to manipulate. Accordingly, these fiber cathode devices suffer many disadvantages that limit their usefulness.